The present invention relates to a dot gain calibration method and apparatus and, more particularly, but not exclusively to dot gain calibration for use with a pre-press system, press or proofing printer.
Dot gain on a printed sheet is an increase in halftone dot size in comparison with dot size on the printing plate. The magnitude of the dot gain depends on factors such as the ink absorption characteristics of the paper, the ink, the screening parameters and the press variables.
The total measured dot gain on the paper is a combination of two separate and unrelated gain factors; plate-related dot gain and press-related dot gain factors. The two gains may be dealt with in any calibrating process to compensate for the dot gain, and may be considered either separately or jointly. Plate related dot gain typically results from chemistry or from the plate exposure process. Press related dot gain, on the other hand, may result from factors such as paper type, screen frequency/geometry, solid ink density/ink characteristics, or press machine attributes such as plate to blanket pressure or blanket characteristics. The data on the plate is considered as constant, while press factors as well as paper alkali/acidity may change over time.
Plate related dot gain is conventionally corrected separately from the press related factors by using correction curves, also called ex-curves (i.e. expose curves). Press-related dot gain can subsequently be compensated for by a process known as tone reproduction.
Tone reproduction is a process in which tonal range is manipulated in order to yield aesthetically pleasing results on printed sheets. Target values are applied for each set of printing parameters and then compensation is made for differences between the actual, that is measured (without compensation), values and the target values (that produce the most pleasing result).
Current tone reproduction processes generally provide reasonable compensation for press-related dot gain, provided that conditions remain stable between compensation and print run.
The current workflow for building a calibrated tone reproduction curve comprises:
Using a prepress system and a CTP (Computer to Plate) or CTF (Computer to Film) machine to create dedicated four plates, one for each color, with test patterns for measurement of the dot gain for different file values for each of the four colors;                2. Loading the plates onto a press and performing a trial print run;        3 Measuring dot percentage values on the printed sheet;        4. Comparing the measured values to a set of predefined required (target) values;        5. Creating a tone reproduction curve to compensate for the differences between the required and measured values; and        6. Producing plates with the job to be printed, using the tone reproduction curve to modify the digital data.        
The above cycle is very expensive and time consuming and the curve is only valid as long as conditions remain exactly the same. If a new paper stock is used, or a slightly different ink formula is placed in the machine, or different screen parameters are used (that is parameters for setting dot patterns to give the color picture) then the entire tone reproduction process has to be repeated. In order to avoid the need for carrying out such calibration, printers often try not to use new paper stock or less predicable kinds of paper such as recycled paper stock. Printers also try to avoid changing to higher quality screen sets, thus saving valuable press and make-ready time by working with familiar dot gains.
Another common technique used in press sites due to the high cost of the current calibration cycle is on-press compensation. Use of non-optimized tone reproduction curves for existing printing conditions and manual compensation of dot gain, performed on press by means of ink keys setup are common. This is a difficult and time consuming task, that becomes even more difficult and almost impossible as use of high screen ruling and FM screening gains popularity.
Even when a dot gain measurement cycle has been performed for a given paper and screen set, the validity of the tone reproduction curve is limited to a time frame, as paper and press characteristics often change over time. Likewise, other factors tend to make themselves felt such as routine replacement of the blankets on the press.
Furthermore, conventional calibration methods are manual, and therefore prone to operator mistakes. There is no closed-loop process of feed back from press to CTP (Computer to Plate) or CTF (Computer to Film) devices and therefore the accurate reproduction of color cannot be fully automated.
Other processes hindered by the complex setup workflow include remote plate-making, where the same digital data serves for manufacturing plates in different press locations and for different presses and different paper stocks. It is quite common that a print job is defined in one location and the data is then sent to a second remote location where the plate is made up and the print run is carried out. It is also quite possible for the same print job to be carried out with remotely created plates at several locations at the same time. Currently, it is difficult to make sure that the different locations accurately print the same color.
Remote preparation is very common in Gravure processes, which use copper cylinders to transfer ink to the paper. There is only a limited number of sites in the world which have the capability to make such cylinders. This requires, therefore, knowledge of printing conditions in remote sites.
A further kind of printing that is not suited to the above process is printing using direct imaging presses. Direct imaging presses use direct-imaging waterless printing plates with sets of ink for which the existing dot-gain calibrations are unsuited.
In the past, screen rulings of 153–200 lines per inch (LPI) have been used. Today, the trend is for greater densities, in the realm of 250+LPI, with dots having circumferences in the range of tens of microns. As will be appreciated, the same percentage dot gain has a greater visual effect the more densely packed are the dots, and thus effective control of dot gain is rendered more and more important.
With larger screen cells and lower screen rulings it was always possible to manipulate the press directly by operating the ink keys. However, with current trends this becomes more difficult.
It is noted that screening techniques can be divided into two. In conventional screening, known as AM, dot distribution is fixed and tones are decided by dot size, or absence of the dot altogether. The alternative is FM screening, in which the dot size is fixed, and the positioning of the dots at greater or lesser frequency is altered. Due to the non-linearity of the visual effect with dot packing, good dot gain control is especially important for FM screening.
Good control of dot gain is especially important for the commercial packaging market. Companies often rely on consumers recognizing the colors on their packaging and do not wish different batches to have different colors. Aside from the consumer issue, exact colors on packaging act as a barrier to product piracy and forgery in general, since it requires expertise and investment on the part of the forger to match colors exactly.
U.S. Pat. No. 5,748,330 to Wang, discloses a technique for calibrating the hardware and software of a digital printing apparatus, which relies on making seven component test patches which completely characterize the printing system. The technique is specific to a grid-based screening technique described in U.S. Pat. No. 5,469,267, which allows use of a function based on the Yule-Neilsen equation and is hence not of general applicability.
There is a need for a robust, automatic and generally applicable tone reproduction measurement and calibration process to overcome the disadvantages of the existing processes.